Researchers from the Monell Center in Philadelphia found exposure to secondhand smoke lowers kids' sensitivity to irritants that would normally make them cough. And because these children's lungs aren't protected through coughing, they are at greater risk for pneumonia, bronchitis and other respiratory diseases.

Despite these increased risks, the study showed children exposed to secondhand smoke may be more likely to develop a smoking habit because experimentation with cigarettes may be less unpleasant for them.

"Cough protects our lungs from potentially damaging environmental threats, such as chemicals and dust. Living with a parent who smokes weakens this reflex, one of the most vital of the human body," study co-director Julie Mennella, a developmental biologist at Monell, said in a news release from the Center.

For the study, the researchers had 38 healthy children, ranging in age from 10 to 17, inhale increasing concentrations of capsaicin (the burning ingredient in chili peppers) from a nebulizer to make them feel like they had to cough. Of the participants, 17 were regularly exposed to secondhand smoke at home and 21 were never exposed to smoke at home. The children's parents were also tested.

The researchers kept increasing the amount of capsaicin the participants inhaled until the person coughed twice. Once that happened, the level of the irritant was recorded as the study participant's cough threshold.

Children exposed to secondhand smoke needed twice as much capsaicin to trigger coughing as the kids who were not exposed to secondhand smoke. Parents who were exposed to smoke were also less sensitive to the irritating cough stimulants.

"This study suggests that even if an exposed child is not coughing, his or her respiratory health may still be affected by secondhand smoke," study co-director Paul Wise, a sensory scientist at Monell, said in the news release.

Future research will investigate if reduced sensitivity to irritants makes smoking more pleasant for teenagers, and whether or not an impaired cough reflex can be reversed, the study authors said.

Sixty percent of American children aged 3 to 11, as well as 18 million young people aged 12 to 19, are exposed to secondhand smoke on a regular basis, the researchers noted.

The findings were published Aug. 20 in the journal Tobacco and Nicotine Research.